The way John Landis remembers it, only actor John Vernon grasped the significance of “Animal House” while it was being made. The director himself had his hands full between shooting the film and arguing with suits at Universal, where it had the support of only two executives, Sean Daniel and Thom Mount. But the veteran character actor, who was playing his first comic role as the Nixon-like Dean Wormer after a career of serious parts in such films as “Point Blank” and “The Outlaw Josey Wales” (the film that led to his casting), was paying attention.

He took me aside after about three weeks of filming,” Landis, 67, says. “He said, ‘John, I don’t think you understand, what we’re doing is important. We’re making a great and important classic film.’ And I thought, ‘John, go back to your trailer.’ I just thought he was crazy.”

Vernon nailed it. The movie with the slim $2.3 million budget — “the cheapest Universal movie in six years,” according to Landis — and a cast made up of “Saturday Night Live” breakout star John Belushi and mostly unknowns went on to become one of the top-grossing movies of 1978, kick-started the college fraternity comedy genre and is now preserved in the National Film Registry.

As Vernon predicted, “Animal House” has become a classic, and it is about to turn 40. In celebration, SF Sketchfest is hosting a 40th anniversary reunion on Sunday, Jan. 21, at San Francisco’s Castro Theatre. Landis and stars Tim Matheson, Peter Riegert, Bruce McGill, James Widdoes, Martha Smith and Mark Metcalf will be on hand for a screening and onstage conversation.

“‘Animal House’ obviously struck a chord and still strikes a chord,” Landis says. “The only remarkable thing about ‘Animal House’ to me is how many people over the years — and I mean over 1,000 people — have come up to me to tell me it was based on their fraternity. It was them. It was their school.

Landis was originally brought in to supervise the rewrite of Harold Ramis, Douglas Kenney and Chris Miller’s script, which was based on their own experiences as college students in 1962. Landis remembers the original script as being the funniest thing he had ever read, but he quickly identified a problem with it. The story, which revolves around the antics of the Delta and Gamma fraternities, was all bad guys. Landis insisted it needed good guys, and so the Deltas were recast as lovable — and egalitarian — rebels and misfits.

“The Omegas were the ones that were exclusive, and we were the Deltas — we had to let anybody in,” says Tim Matheson. who was originally offered the role of a Gamma, but held out for the role of Delta rush chairman Otter. “You apply, you get in, what the hell. ‘We’re not like those idiots. We’re not like those elitist, sexist, racist idiots.’ To me, that’s what made the movie special.”

Matheson, who has been acting since childhood, regards “Animal House” as his first really adult film as well as his first comedy. He remembers being surprised to discover that Landis was younger than he was but impressed by the director’s confidence and deep knowledge of film. And he credits him for setting just the right tone as they worked far from Universal executives’ prying eyes on location in Oregon.

“He infused the set with a spirit of adventure and freedom, yet he would — he was smart,” Matheson, 70, says. “We could only print one take. The studio wouldn’t allow us to shoot multiple takes, so John would never cut. He would just scream at us. He’d go, ‘Do it again! Do it again! And do it funnier!’ It was outrageous and fun and anarchy. He imbued the set with that, which was the spirit of the film.”

What Landis couldn’t see as a young man when the middle-aged Vernon told him they were making something very special he has come to understand.

He realizes now that “Animal House” is a stand-in for so many people’s experiences of leaving home for the first time.

“That sense of freedom,” he says. “It’s that time of your life when you’re an adult physically, but not mentally. It’s that sense of freedom and sense of adventure and joy, that’s when people make many stupid decisions. The bottom line is ‘Animal House’ captures that somehow.”